This invention relates to the design, synthesis and application of novel fluorinated boron based compounds which act as anion receptors in non-aqueous battery electrolytes. As a result, the anion receptors of the present invention can be used as additives to enhance the ionic conductivity and cation transference number of non-aqueous electrolytes. More specifically, the family of anion receptors of the present invention includes phenyl boronate, fluorinate phenyl boronate, and fluorinated alkyl substituted phenyl boronate based compounds bearing different fluorinated alkyl and aryl groups.
In the past, research has been conducted on the reduction of ion pairing in non-aqueous electrolytes. The design and synthesis of receptor molecules for the selective complexation of ions has been an active area of research in the last two decades. With respect to lithium batteries, ion pairing accounts for the low lithium transference in non-aqueous electrolytes. To decrease ion pairing, researchers have used either solvents of high dielectric constant or added a neutral ligand to coordinate either the cation or the anion. Coordination with either the cation or anion was expected to increase the cation-anion distance of closest approach and thus decrease ion pair formation. For cation coordination in liquid non-aqueous electrolytes, Salomon [1] has reported the use of crown ether 18-crown-6, Matsuda, et al. [2], the use of 12-crown-4, and Schriever, et al. [3], have used cryptands to decrease ion pair formation in polymer electrolytes.
More recently, research has focused on providing neutral compounds to complex anions. These compounds were summarized in a review article written by F. P. Schmidtchen and M. Berger [4]. However, because anion complexation occurs through hydrogen bonding, these neutral compounds cannot be used in lithium batteries. Lee, et al. have utilized aza-ether based compounds as anion receptor molecules. Electron withdrawing groups were used to substitute amine hydrogen atoms in linear aza-ethers, multi-branched aza-ethers, and cyclic aza-crown ethers [5].
U.S. Pat. No. 5,849,432 to Angell et al. discloses liquid boron-containing electrolyte solvents and liquid boron-containing electrolyte solutions, wherein the backbones of the boron compounds have either an O3B or O2B-X structure, where X is limited to a halogen atom. However, Angell et al. do not disclose boron compounds wherein the backbone structure is O2BR, where R is a phenyl group.
Anion coordination is much more important than cation coordination in lithium battery electrolytes because it offers the ability of increasing both conductivity and lithium transference number. Accordingly, there is still a need in the art of lithium batteries for electrolyte additives which can complex anions, yet are stable in lithium batteries. There is also a need in the art of lithium batteries to enhance the conductivity of inexpensive and environmentally friendly inorganic salts such as LiF, LiCl, LiBr and LiI. In addition, there is a need to increase the transference number of the Li+ ion. In many non-aqueous electrolytes, in particular polymer electrolytes, the transference number of the Li+ ion is low. This introduces additional polarization losses in batteries and reduces the utilization of the cathode material.
It is therefore, an object of the present invention to provide a new family of compounds which enhances the conductivity of lithium battery electrolytes by complexing with the anion moiety of the salt, and also increases the transference number of the Li+ ion in electrolytes.
Another object of the present invention is to increase the conductivity of cost effective electrolyte salts such as LiF, LiCl, LiBr and LiI.
Another object of the present invention is to provide improved electrochemical cells by use of electrolyte additives.
The present invention, which addresses the needs of the prior art, provides novel fluorinated boron-based compounds which act as anion receptors in non-aqueous battery electrolytes. When added to non-aqueous electrolytes, the receptors of the present invention complex the anion moiety of the electrolyte salt, thereby increasing the conductivity of the electrolytes and the transference number of Li+ ion in electrolytes. The present invention also relates to the use of fluorinated boron-based anion receptors as electrolyte additives for both primary and secondary lithium batteries. Electrolytes used for the electrochemical cells of the present invention include liquid electrolytes using organic solvents, polymer electrolytes, and gel electrolytes.
As a result of the present invention, stable anion receptor compounds are provided which increase dramatically the conductivity of electrolytes for lithium batteries. The electrolyte conductivity is increased because the fluorinated boron-based compounds of the present invention complex anion moieties in non-aqueous electrolytes thereby increasing the concentration of lithium cations available for transport. As a result of using the anion receptors of the present invention, lithium batteries are provided which have significantly increased rate capability or discharge current density. The enhanced batteries of the present invention also have increased cathode utilization because of the increased Li+ ion transference number.
The anion receptors of the present invention include a boron-based compound of the formula QO2BR or (QO)2BR, wherein R is a phenyl, a fluorinated phenyl, or a fluorinated alkyl substituted phenyl and Q is a fluorine bearing moiety. R is selected from the group consisting of C6H5, C6H4F, C6H3F2, C6H2F3, C6HF4, C6F5, C6H4CF3, and C6H3(CF3)2 and Q is selected from the group consisting of xe2x80x94C6H3Fxe2x80x94, xe2x80x94C6H2F2xe2x80x94, xe2x80x94C6HF3xe2x80x94, xe2x80x94C6F4xe2x80x94, xe2x80x94((CF3)2C)2xe2x80x94, xe2x80x94C6F5, and xe2x80x94(CF3)2CH.
A preferred embodiment of the present invention is an electrochemical cell which includes a non-aqueous electrolyte solvent and an electrolyte additive that includes a boron-based anion receptor, wherein said boron-based anion receptor is a compound having the formula QO2BR or (QO)2BR, wherein R is a phenyl, a fluorinated phenyl, or a fluorinated alkyl substituted phenyl and Q is a fluorine bearing moiety. R is selected from the group consisting of C6H5, C6H4F, C6H3F2, C6H2F3, C6HF4, C6F5, C6H4CF3, and C6H3(CF3)2 and Q is selected from the group consisting of xe2x80x94C6H3Fxe2x80x94, xe2x80x94C6H2F2xe2x80x94, xe2x80x94C6HF3xe2x80x94, xe2x80x94C6F4xe2x80x94, xe2x80x94((CF3)2C)2xe2x80x94, xe2x80x94C6F5, and xe2x80x94(CF3)2CH. The non-aqueous electrolyte solvent for the electrochemical cell is selected from the group consisting of tetrahydrofuran, 2-methyl furan, 4-methyldioxolane, 1,3-dioxolane, 1,2-dimethoxyethane, dimethoxymethane, ethylene carbonate, propylene carbonate, xcex3-butyrolactone, methyl formate, sulfolane, acetonitrile and 3-methyl-2-oxazolidinone, dimethyl carbonate, dimethyl ether, 1-methyl-2-pyrrolidinone and poly(ethylene oxide). In one embodiment, the non-aqueous electrolyte solvent is a gel electrolyte selected from the group consisting of poly(acrylo nitrile) and poly(vinylidene flouride-hexafluoro propylene).
The electrochemical cell can include a lithium salt in a liquid organic solvent wherein the lithium salt is selected from the group consisting of LiF, LiCl, LiBr, LiI, CF3COOLi, C2F5COOLi, C6F5COOLi and mixtures thereof. In a preferred embodiment, the liquid organic solvent is a compound selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, dimethyl ether, xcex3-butyrolactone, 3-methyl-2-oxazolidinone, 1-methyl-2-pyrrolidinone and mixtures thereof.
In another embodiment, the electrochemical cell includes an electrolyte solute selected from the group consisting of LiF, LiCl, LiBr, LiI, CF3COOLi, C2F5COOLi, and mixtures thereof.
A preferred embodiment of the electrochemical cell includes an anode selected from the group consisting of lithium, lithium alloys, lithium carbon intercalation compounds, lithium graphite intercalation compounds, lithium metal oxide intercalation compounds, and mixtures thereof. The electrochemical cell can also include a cathode selected from the group consisting of a transition metal oxide, a transition metal chalcogenide, a poly(carbon disulfide) polymer, an organo-disulfide redox polymer, a polyaniline, an organo-disulfide/polyaniline composite and an oxychloride. The transition metal oxide is selected from the group consisting of Li2.5V6O13, Li1.2V2O5, LiCoO2, LiNiO2, LiMn2O4, LiMnO2, LiNi1xe2x88x92xMxO2, (Mxe2x95x90Co, Mg, Al, and Ti); the transition metal chalcogenide is selected from the group consisting of Li3NbSe3, LiTiS2 and LiMoS2; the organo-disulfide/polyaniline composite is a mixture of polyaniline and 2,5 dimercapto-1,3,4-thiadiazole; and the organo-disulfide redox polymers are formed by reversible electrochemical dimerization/scission or polymerization/depolymerization of organo disulfide polymers by the reaction:
xe2x80x94(Sxe2x80x94Rxe2x80x94S)nxe2x80x94+2nexe2x88x92=nSxe2x88x92xe2x80x94Rxe2x80x94Sxe2x88x92
wherein R is an aliphatic or aromatic entity and n greater than 50.
A preferred embodiment of the present invention is a method of enhancing the conductivity of a non-aqueous battery electrolyte which includes adding to said electrolyte a conductivity enhancing amount of a fluorinated boron-based anion receptor. Preferably, the anion receptor is selected from the group consisting of C6H5, C6H4F, C6H3F2, C6H2F3, C6HF4, C6F5, C6H4CF3, C6H3(CF3)2, xe2x80x94C6H3Fxe2x80x94, xe2x80x94C6H2F2xe2x80x94, xe2x80x94C6HF3xe2x80x94, xe2x80x94C6F4xe2x80x94, xe2x80x94((CF3)2C)2xe2x80x94, xe2x80x94C6F5, and xe2x80x94(CF3)2CH. In a preferred embodiment, the method also includes adding to the electrolyte an electrolyte solute selected from the group consisting of LiClO4, LiAsF6, LiBF4, LiPF6 and LiSbF6. In another embodiment, an electrolyte solute selected from the group consisting of LiF, LiCl, LiBr, LiI, CF3COOLi, C2F5COOLi, and mixtures thereof is added to the electrolyte.
Another embodiment of the present invention provides a non-aqueous battery electrolyte which includes a solvent; a lithium salt; and an additive that includes an anion receptor having a boron-based compound of the formula QO2BR or (QO)2BR, wherein R is a phenyl, a fluorinated phenyl, or a fluorinated alkyl substituted phenyl and Q is a fluorine bearing moiety. R is selected from the group consisting of C6H5, C6H4F, C6H3F2, C6H2F3, C6HF4, C6F5, C6H4CF3, and C6H3(CF3)2 and Q is selected from the group consisting of xe2x80x94C6H3Fxe2x80x94, xe2x80x94C6H2F2xe2x80x94, xe2x80x94C6HF3xe2x80x94, xe2x80x94C6F4xe2x80x94, xe2x80x94((CF3)2C)2xe2x80x94, xe2x80x94C6F5, and xe2x80x94(CF3)2CH. In a preferred embodiment, the solvent is selected from the group consisting of tetrahydrofuran, 2-methyl furan, 4-methyldioxolane, 1,3-dioxolane, 1,2-dimethoxyethane, dimethoxymethane, ethylene carbonate (hereinafter xe2x80x9cECxe2x80x9d), propylene carbonate, sulfolane, xcex3-butyrolactone, methyl formate, acetonitrile and 3-methyl-2-oxazolidinone, dimethyl carbonate (hereinafter xe2x80x9cDMCxe2x80x9d), dimethyl ether (hereinafter xe2x80x9cDMExe2x80x9d), 1-methyl-2-pyrrolidinone and mixtures thereof and poly(ethylene oxide). In one embodiment, the solvent is a gel electrolyte selected from the group consisting of poly(acrylo nitrile) and poly(vinylidene flouride-hexafluoro propylene). The lithium salt for the non-aqueous battery electrolyte is selected from the group consisting of LiF, LiCl, LiBr, LiI, CF3COOLi, C2F5COOLi, C6F5COOLi and mixtures thereof.
Another embodiment of the present invention is a non-aqueous battery electrolyte additive which includes an anion receptor having a boron-based compound of the formula QO2BR or (QO)2BR, wherein R is a phenyl, a fluorinated phenyl, or a fluorinated alkyl substituted phenyl and Q is a fluorine bearing moiety. R is selected from the group consisting of C6H5, C6H4F, C6H3F2, C6H2F3, C6HF4, C6F5, C6H4CF3, C6H3(CF3)2 and Q is selected from the group consisting of xe2x80x94C6H3Fxe2x80x94, xe2x80x94C6H2F2xe2x80x94, xe2x80x94C6HF3xe2x80x94, xe2x80x94C6F4xe2x80x94, xe2x80x94((CF3)2xe2x80x94, xe2x80x94C6F5, and xe2x80x94(CF3)2CH. In one embodiment, the non-aqueous battery electrolyte additive can also include an electrolyte solute selected from the group consisting of LiClO4, LiAsF6, LiBF4, LiPF6 and LiSbF6. In another embodiment, the non-aqueous battery electrolyte additive can include a lithium salt selected from the group consisting of LiF, LiCl, LiBr, LiI, CF3COOLi, C2F5COOLi, and mixtures thereof.
When added to liquid, non-aqueous electrolytes containing salts such as LiF, LiCl, LiBr or LiI, CF3COOLi, and C2F5COOLi, the fluorinated boron-based compounds of the present invention provide a salting-in effect which results in increased solubility and electrolyte conductivity. Thus, another important advantage of using the boron-based compounds of the present invention is the significant cost savings resulting from using low cost electrolyte salts such as LiF, LiCl, LiBr and LiI.
Other improvements which the present invention provides over the prior art will be identified as a result of the following description which sets forth the preferred embodiments of the present invention. The description is not in any way intended to limit the scope of the present invention, but rather only to provide a working example of the present preferred embodiments. The scope of the present invention will be pointed out in the appended claims.